Grouping objects Arrays, Collections and Iterators 1.0

2 Main concepts to be covered Arrays Collections Iterators

3 Requirement to group objects Many applications for collections of objects: –Personal organizers –Library catalogs –Student-record system The number of items to be stored varies: –Items added –Items deleted

4 Fixed-size collections Programming languages usually offer a special fixed-size collection type: an array Arrays are built-in, use [] syntax Java arrays can store objects or primitive-type values Maximum collection size must be fixed at Array creation time How is Array creation time more dynamic than in other programming languages, such as C++?

5 Creating an array object public class LogAnalyzer { private int[] hourCounts; private LogfileReader reader; public LogAnalyzer() { hourCounts = new int[24]; reader = new LogfileReader(); }... } Array object creation Array variable declaration

6 The hourCounts array

7 Using an array Square-bracket notation is used to access an array element: hourCounts[hour] Elements are used like ordinary variables –In an expression: adjusted = hourCounts[hour] – 3; hourCounts[hour]++;

8 Class libraries Collections of useful classes Encourages reuse of design and code Java organizes its libraries in packages The java.util package includes classes for grouping objects in collections

9 A personal notebook Notes may be stored No limit to the number of notes It tells how many notes are stored

10 import java.util.ArrayList; /** *... */ public class Notebook { // Storage for an arbitrary number of notes. private ArrayList notes; /** * Perform any initialization required for the * notebook. */ public Notebook() { notes = new ArrayList(); }... }

11 Object structures with collections

12 Features of ArrayList It keeps the objects in order It increases its capacity as necessary It keeps a private count: size() accessor retrieves it

13 Using ArrayList collection public class Notebook { private ArrayList notes;... public void storeNote(String note) { notes.add(note); } public int numberOfNotes() { return notes.size(); }... } Adding a new note Returning the number of notes (delegation).

14 Adding a third note myBook.add(“11:30 meet John”)

15 Retrieving an object Retrieve and print the note public void showNote(int noteNumber) { if(noteNumber = numberOfNotes()) { // This is not a valid note number. } else System.out.println(notes.get(noteNumber)); }

16 Removal may affect numbering myBook.remove(1)

17 Review: ArrayList Items may be added and removed Each item has an index Index values may change if items are removed or further items added The main ArrayList methods are add, get, remove and size For more methods, see API documentAPI document

18 Collections framework Goals comparable to C++’s STL Data structures: –Vector, LinkedList, HashSet, etc. Algorithms: –Sorting, searching, shuffling, etc.

19 Loops and bugs Why are loops often a cause of bugs? What kind of bugs? Wouldn’t be nice to avoid these bugs?

20 Iterating over a collection Iterator it = myCollection.iterator(); while(it.hasNext()) { call it.next() to get the next object do something with that object } java.util.Iterator Returns an Iterator object public void listNotes() { Iterator it = notes.iterator(); while(it.hasNext()) { System.out.println(it.next()); } How does Iterator help avoid bugs?

21 Review: loops and Iterator Java while and for loops are similar to C++ –Similar bugs, too! Collection classes have special Iterator objects that simplify iteration over the whole collection –Bounds-checking methods help avoid bugs